High intensity focused ultrasound (HIFU) can be used to treat various pathologies within the uterus. HIFU ablation of fibroids and adenomyosis have been reported, and various additional procedures may also be amenable to HIFU treatment, including endometrial ablation, ablation of polyps, et al.
When HIFU is applied transabdominally, it is important to ensure that the patient's bowel does not intervene in the beam path. Otherwise, the gas that is naturally present in bowel will tend to reflect the HIFU, resulting in reduced treatment efficacy and/or dangerous burns of the bowel tissue. If transabdominal imaging ultrasound is used to guide the HIFU treatment, bowel gas will tend to block its transmission as well, resulting in poor quality images of the uterus. The anatomical aperture through which ultrasound may pass safely and unimpeded by the bowel is herein termed the “acoustic window”.
In addition, the target uterine tissue must remain within a treatable depth range, i.e. at a depth below the skin surface that is within the focal range of the HIFU and/or imaging transducers (which are typically limited by practical design considerations).
In an attempt to increase the acoustic window, users of commercially-available HIFU systems have reported catheterizing the patient's bladder and filling it with fluid to help displace bowel laterally away from the HIFU beam path. In doing so, the acoustic window becomes defined largely by the lateral dimensions of the bladder. Ultrasound is readily transmitted through the bladder (i.e. since the fluid is relatively transparent to ultrasound), but not through the surrounding bowel.
Yet as the bladder fills and expands laterally, it also tends to expand posteriorly away from the abdominal wall. This pushes the underlying uterus in the posterior direction as well, since the bowel and rectum behind the uterus tend to be easily compressed. As the uterus is pressed deeper by the filling bladder, it may eventually move out of range of the abdominally-placed ultrasound or HIFU transducer. Thus, filling the bladder can actually be counter-productive when used as a standalone technique for enlarging the acoustic window.
Furthermore, as the bladder fills, it assumes a more spherical shape and becomes less pliable. This results in a decreased contact area at the interfaces of the bladder-uterus and bladder-abdominal wall, allowing loops of bowel to slip in between these interfaces from the periphery.
Given these problems, there is a need for a technique that can reliably increase the size of the acoustic window by displacing the bowel from the intended beam path, while minimizing the required focal depth to reach uterine target tissue.